The invention relates to an improved double talk detector for controlling convergence of an echo canceller, and to a method of controlling convergence of an echo canceller.
In a telephone network, four wire (4W) and two wire (2W) segments are joined at opposite ends of the network bu hybrid circuits, often called 4:2 hybrids. Impedance mismatch in a hybrid circuit causes a 4W receive signal to be reflected back onto the 4W transmit path. If there is enough delay in the network, this reflected signal presents itself as echo to the speaker who originated it at the far end. Adaptive echo cancellers remove the echo signal from the 4W transmit path.
Normally, a 4W receive signal is at a higher level than its echo signal on the 4W transmit path, since there is loss across the hybrid circuit. Near end speech on the trransmit path will therefore typically be stronger than the echo signal. Such near end speech is unwanted noise as far as convergence of the echo canceller is concerned, since it will diverge the canceller if the canceller were to continue updating its estimated impulse response while near end speech is present. Consequently, a critical component of an echo canceller is a near end speech detector, or double talk detector, to inhibit updating of the estimated impulse response while near end speech is present. To detect near end speech on the 4W transmit path, it is not sufficient to simply look for energy on the path, since it will be there from echo even when there is no near end speech. Various techniques have therefore been developed to detect near end speech and inhibit updating of the canceller when it is present.
A performance measure that represents the effectiveness of the cancellation process, such as echo return loss enhacement (ERLE), can be used to detect double talk. In single talk, when there is only far end speech, an adaptive filter of the canceller can cancel most of the energy on the 4W transmit in path, leaving little residual echo on the 4W transmit out path. Thus, when only far end speech is present, the ratio of the 4W transmit in energy to 4W transmit out energy, i.e., the ERLE, is high and much greater than one. When near end speech is present, however, the adaptive filter can cancel only a small portion of the 4W transmit in energy, allowing the near end speech to pass as residual energy. Under this condition, the ERLE is low and approaches one. The double talk detector can then simply monitor the ERLE, and when the ERLE is high, allow the taps or estimated impulse response to update to converge the adaptive filter. Should the ERLE begin to decrease, then the taps are frozen to prevent divergence of the filter.
The double talk detection method of measuring the ERLE provides satisfactory convergence control in a nonswitched environment, but suffers from a fundamental limitation in a switched environment where the end path of the telephone network may change from one call to the next. An echo canceller's ability to remove echo from a 4W transmit path depends upon it being able to generate an accurate model of the end path impulse response. When the end path changes, the canceller can no longer match the echo, because the model it developed of the previous end path does not accurately represent the impulse response of the new end path. The result is poor cancellation, which causes the ERLE to drop to near unity and appear as near end speech to the double talk detector, even when near end speech is not present. This, in turn, causes the detector to inhibit the echo canceller from converging and developing an accurate model of the new end path impulse response.